internal/postgres: get next modules to fetch in one query

Construct a single query that returns all the modules we want to
reprocess, in the order we want.

The main ideas are:

- Use a WITH statement (CTE) to construct a table of (module path,
  latest version).

- Add a computed "latest" column to every row by testing membership in
  that table.

- Use a CASE in the ORDER BY clause to express the ordering we want.

Some other changes:
- We process status 0 first (the original motivation for this CL).

- As a consequence of using one query, we'll always return a number of
  results. up to the limit.  To handle the lower limit for large
  modules, we add code to truncate the results.

- Before, all 5xx that were not one of the four special codes were
  processed after everything else. Now it's relatively easy to process
  other 5xx codes at the end of each category (latest versions,
  non-large modules).

Change-Id: I4dacb093cfb93299ccfc8d29c1fd0b1cb49bf56f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/239479
Reviewed-by: Julie Qiu <julie@golang.org>

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